Thermistors, diodes, and the like are known as encapsulated-semiconductor parts. A thermistor, for example, is an encapsulated-semiconductor part in which the semiconductor has the property of changing in electrical resistance with increasing temperature. By taking advantage of this property and measuring the electrical resistance of the semiconductor, temperature can be determined with the part.
In particular, as shown in the sole prior art FIGURE, a thermistor 10 called a bead thermistor or glass thermistor includes a semiconductor (thermistor chip) 1, lead wires 2, and a semiconductor encapsulation material 3. The thermistor chip 1 and part of the lead wires 2 have been coated and encapsulated with the semiconductor encapsulation material (glass for metal coating) 3, so that the thermistor 10 can be used at high temperatures and in oxidizing atmospheres. Examples of the thermistor chip 1 include oxide materials and non-oxide materials constituted of at least one member selected from the group consisting of nitrides, carbides, borides, and silicides, and oxide materials are in extensive use mainly from the standpoint of properties or cost. As the lead wires 2, Dumet wires (Fe—Ni alloy coated with Cu) and the like are used.
Such a glass for metal coating is required to satisfy, for example, the following: (1) to have a thermal expansion coefficient which matches with those of the lead wires and semiconductor chip so that the encapsulation of the lead wires and the semiconductor chip does not result in cracking; (2) to be usable in encapsulation at a temperature lower than the heat resistance temperature of the lead wires; and (3) not to contain any harmful substance, e.g., lead.
Hitherto, borosilicate glasses have been proposed as glasses for metal coating which meet such requirements (see, for example, JP-A-2002-37641 and JP-A-2003-17632). Furthermore, an aluminosilicate glass (e.g., WO 2006/35882) has been proposed as an encapsulation glass for thermistors to be used at high temperatures.
In recent years, a heat source in, e.g., a building or the combustion system of a power generator have been required to be kept in an optimal operating state in order to minimize CO2 or NOx, emission in environmental measures for carbon dioxide reduction and acid rain prevention or from the standpoint of ensuring safety. For optimizing the state in which a gas, oil, or the like burns in the heat source or combustion system, it is necessary to directly monitor the temperature in the combustion chamber and automatically control the heat source or combustion system. The thermistors to be used in this application must have a heat resistance temperature of about 600° C.
However, since glass thermistors employing the glasses described in JP-A-2002-37641 and JP-A-2003-17632 have low heat resistance and these glasses for metal coating soften or deform upon exposure to an atmosphere of 500 to 600° C., these glasses cannot hence be used in such applications.
On the other hand, the glass for metal coating described in WO 2006/35882 has been designed to be applied to lead wires made of platinum, and encapsulation therewith is conducted at a temperature as high as above 1,000° C. Because of this, when a metal susceptible to oxidation, e.g., Dumet, is used as lead wires, the lead wires are oxidized to form a brittle oxide on the surface thereof. This oxide is apt to be peeled off from the metal and, hence, the encapsulated part readily deteriorates. Consequently, such metals cannot be used.